This invention relates to a method and apparatus for the continuous preparation of animal feed pellets and, more particularly, to a method employing a unique pressure cooking process. Various cereal grains, preferably in milled form, plant and animal proteins, roughage products, liquids, and other miscellaneous ingredients have been mixed together and conditioned to form a mash. Most pellet producing processes include a hopper or holding bin containing the mash, a conditioner in which moisture and heat are applied to the mash, and a pellet producing means such as a roller and die apparatus into which the mash is fed from the conditioner. Means such as paddle conveyors, screw feeders, chutes and the like are employed to move the material from the hopper, through the conditioner and into the die area.
A pellet mill installation is generally in an environment with an uncontrolled temperature such that when the mash issues from the conditioning chamber into an atmospheric pressure downspout, the mash approaches ambient temperature which can vary the characteristics of the pellets. Means for supplying and controlling heat, moisture and pressure in the conditioning chamber are also included in most systems. The moisture content may be controlled by adjusting the amount of steam, water, dry heat or some other suitable moisture controlling ingredient. The moisture content of the mash may be automatically set within a prescribed range, and the feed rate of the mash may be automatically controlled in response to the moisture content as taught by Volk U.S. Pat. No. 3,932,736.
Because of the different ingredients and various formulas of animal food, different rates of mash introduction and different amounts of moisture addition are required. Further, when the pelleting mash is conditioned with steam under atmospheric pressure, the different formulations require different temperatures and moisture content for proper conditioning. This results in temperatures of the mash exiting the conditioner varying from 70.degree. F. to 212.degree. F. and an added moisture variation from 0% to 8%.
In the prior art, the die or pellet producing means is operated by motors which may be anywhere from 25 to over 600 horsepower. Extruding the mash through the die is a complex rheological process of deformation and flow conditioned upon stress, strain and time. The load on the die and, thus, the efficiency of the system and the quality of the pellets produced is primarily dependent on two factors: (1) the rate at which the grain mash is fed to the die area; generally, the faster the feed rate, the greater the load, and (2) the composition of the mash, its moisture and temperature content, and the degree of its gelatinization and deaeration.
Improving the starch gelatinization of the mash results in decreased horsepower requirements and other advantages including better pellet quality and increased feed efficiency. However, the conventional pelleting systems, such as that taught by U.S. Pat. No. 4,001,452, are limited in terms of high-intensity conditioning and hence starch gelatinization. As described in E. Heidenreich, Operation Strategies for Expansion Cooking, 2(5) Feed Mix 32 (1994), the addition of steam of more than 3% in conventional systems causes problems for the pellet pressing process, and results in an increase in temperature of only approximately 50.degree. C. which is too short-lived to create the desired conditioning effects. Further, in conventional devices, high fat content acts as a barrier to moisture thereby obstructing its diffusion into the mash and resulting in poor pellet quality.
Consequently, as described by Heidenreich, supra, the limited intensity level of conventional pelleting in terms of moisture, temperature and pressure results in problems such as insufficient hygienization (especially for the decontamination of salmonella), inadequate breakdown of antinutritive substances, fines, poor bioavailability of proteins and other nutrients, and a difficulty in manufacturing pellets containing high fat levels.
Due to these shortcomings, expanders, such as those manufactured by Sprout-Matador Co., Inc. of Muncy, Pa. and Amandus Kaul of Hamburg, Germany, have been developed which mechanically stress the mash by shearing and thermally stress the mash by friction and dissipation of the mechanical energy. Use of an expander results in enhanced nutritional value of the feed, hydrolysis of the starch, gelatinization, degradation of antinutritional factors, hygienization, and an overall decrease in bulk density. The net result is increased nutritional and sensoric properties of the feed. Further, the hygienic quality of the feed can be raised to the level of foodstuffs with expanders.
Expanders are incorporated into conventional pelleting machines, typically affixed between the distal end of the conditioning chamber and the die and roller apparatus. The length of a typical expander varies from about 5155 mm to about 8790 mm. The mash entering the expander from the conditioning chamber has a temperature of about 70-80.degree. C. and a moisture content of about 16-18% and exits the expander at temperatures between about 90.degree. C. and 135.degree. C. Generally, the maximum temperature to which the mash is exposed is about 140.degree. C. as a result of the expander's physical and mechanical construction. However, the active temperature range for most popular types of raw materials is lower. The temperature selected depends on the raw material selected and the desired influence. Too high a temperature can break down valuable amino acids, vitamins, pharmaceutical additives or other components.
At high temperatures, neutral fats or free fatty acids are not damaged. Expansion has a positive effect on the fat quality during storage of the feed because the lipases and lipoxydases that oxidize fats are thermally inactivated. Further, the fat is more bioavailable and accessible for digestive enzymes. According to M. Peisker, Influence of Expansion of Feed Components, 2(3) Feed Mix 26 (1994), this improvement in fat digestibility is probably the main reason for an increase in available calories, which can be between 2% and 4% depending on the composition selected.
As discussed in Peisker, supra, the modification of starch during expansion has been examined extensively. Physical changes, or gelatinization, consist of a transfer from a crystalline state to the amorphous state. The expander's shearing force essentially ruptures the starch molecules and tends to cause the feed particles to aggregate into clumps. Gelatinization dramatically improves bioavailability and is generally essential for most animals. For example, gelatinized starch is a more accessible substrate for lactic fermentation in the piglet stomach. Gelatinized starch also enhances pellet quality because of its ability to adhere together and integrate liquids, such as oils, into its pasty structure.
Proteins are temperature sensitive molecules which will, under high temperatures, coagulate or denature. This can be advantageous if the mash, prior to expansion, contains proteins such as enzyme inhibitors, as is possible in the case of peas or insufficiently toasted soybeans. Protein digestibility has been significantly enhanced by use of expanders. Expansion also results in faster glucose influx in the gut after feed consumption. Further, according to Peisker, supra, subjecting the mash to a drastic pressure drop when it leaves the expander results in rupture of cell walls of microorganisms and the grain.
Throughput ranges from between about 1.5 and 30 tons per hour, requiring a total driving power of between 75 and 315 kW per hour. Once the mash exits the expander it is cooled at atmospheric pressure prior to entering the die area where it is extruded into pellets.
As mentioned, the expander is an add-on apparatus that conditions the mash by hydrothermal, pressure and mechanical means. As such, additional costs for the expander, for electrical and hydrothermal energy, and for maintenance are required. The present invention relates to achieving better or similar results in final pellet characteristics as those subjected to the expansion process. The present invention subjects the mash to steam under pressure from the time the mash enters the conditioning chamber until it exits the die as pellets. Applicant is aware of an attempt by Central Soya Co., Inc. of Fort Wayne, Ind. in the late 1970's to make a machine employing pressure all the way through the die area. However, this attempt was unsuccessful and the project and prototype were abandoned after short use.